Chemical vapor deposition (CVD) is a known process in the semiconductor industry for forming thin films of materials on substrates such as silicon wafers. In CVD, reactant gases (also referred to herein as “precursor gases”) of different reactants are delivered to one or more substrates in a reaction chamber. In many cases, the reaction chamber includes only a single substrate supported on a substrate holder (such as a susceptor), with the substrate and substrate holder being maintained at a desired process temperature. The reactant gases react with one another to form thin films on the substrate, with the growth rate being controlled either by the temperature or the amounts of reactant gases.
In some applications, the reactant gases are stored in gaseous form in a reactant source vessel. In such applications, the reactant vapors are often gaseous at ambient (i.e., normal) pressures and temperatures. Examples of such gases include nitrogen, oxygen, hydrogen, and ammonia. However, in some cases, the vapors of source chemicals (“precursors”) that are liquid or solid (e.g., hafnium chloride) at ambient pressure and temperature are used. These source chemicals may have to be heated to produce sufficient amounts of vapor for the reaction process. For some solid substances (referred to herein as “solid source precursors”), the vapor pressure at room temperature is so low that they have to be heated to produce a sufficient amount of reactant vapor and/or maintained at very low pressures. Once vaporized, it is important that the vapor phase reactant is kept at or above the vaporizing temperature through the processing system so as to prevent undesirable condensation in the valves, filters, conduits, and other components associated with delivering the vapor phase reactants to the reaction chamber. Vapor phase reactants from such naturally solid or liquid substances are useful for chemical reactions in a variety of other industries.
Atomic layer deposition (ALD) is another known process for forming thin films on substrates. In many applications, ALD uses a solid and/or liquid source chemical as described above. ALD is a type of vapor deposition wherein a film is built up through self-saturating reactions performed in cycles. The thickness of the film is determined by the number of cycles performed. In an ALD process, gaseous precursors are supplied, alternatingly and repeatedly, to the substrate or wafer to form a thin film of material on the wafer. One reactant adsorbs in a self-limiting process on the wafer. A different, subsequently pulsed reactant reacts with the adsorbed material to form a single molecular layer of the desired material. Decomposition may occur through reaction with an appropriately selected reagent, such as in a ligand exchange or a gettering reaction. In a typical ALD reaction, no more than a molecular monolayer forms per cycle. Thicker films are produced through repeated growth cycles until the target thickness is achieved.
A typical solid or liquid source precursor delivery system includes a solid or liquid source precursor vessel and a heating means (e.g., radiant heat lamps, resistive heaters, etc.). The vessel includes the solid (e.g., in powder form) or liquid source precursor. The heating means heats up the vessel to increase the vapor pressure of precursor gas in the vessel. The vessel has an inlet and an outlet for the flow of an inert carrier gas (e.g., N2) through the vessel. The carrier gas sweeps precursor vapor along with it through the vessel outlet and ultimately to a substrate reaction chamber. The vessel typically includes isolation valves for fluidly isolating the contents of the vessel from the vessel exterior. Ordinarily, one isolation valve is provided upstream of the vessel inlet, and another isolation valve is provided downstream of the vessel outlet. Precursor source vessels are normally supplied with tubes extending from the inlet and outlet, isolation valves on the tubes, and fittings on the valves, the fittings being configured to connect to the gas flow lines of the remaining substrate processing apparatus. It is often desirable to provide a number of additional heaters for heating the various valves and gas flow lines between the precursor source vessel and the reaction chamber, to prevent the precursor gas from condensing and depositing on such components. Accordingly, the gas-conveying components between the source vessel and the reaction chamber are sometimes referred to as a “hot zone” in which the temperature is maintained above the vaporization/condensation temperature of the precursor.
It is known to provide a serpentine or tortuous flow path for the flow of carrier gas while it is exposed to a solid or liquid precursor source. For example, U.S. Pat. Nos. 4,883,362; 7,122,085; and 7,156,380 each disclose such a serpentine path.